The present invention relates to an engine control system of an internal combustion engine with both a variable compression ratio mechanism and an exhaust-gas recirculation (EGR) control system, and specifically to techniques for enhancing fuel economy and engine performance under a part load condition of a spark-ignition internal combustion engine.
In recent years, there have been proposed and developed various reciprocating internal combustion engines equipped with a variable compression ratio mechanism enabling a nominal compression ratio or a geometrical or mechanical compression ratio denoted by Greek letter xcex5 (epsilon) to be continuously varied depending on engine operating conditions. One such variable compression ratio mechanism has been disclosed in Japanese Patent Provisional Publication No. 2000-73804 (hereinafter is referred to as xe2x80x9cJP2000-73804xe2x80x9d). In the internal combustion engine of JP2000-73804, a multi-link type piston-crank mechanism is used as a variable compression ratio mechanism. Under a part load condition, compression ratio xcex5 is adjusted to a high compression ratio in order to enhance a thermal efficiency. In contrast, at high loads, compression ratio xcex5 is set or adjusted to as high a ratio as possible, taking into account the frequency of detonation or knock. On the other hand, Japanese Patent Provisional Publication No. 7-259655 (hereinafter is referred to as xe2x80x9cJP7-259655xe2x80x9d) discloses a variable compression ratio engine capable of switching between a standard Otto-cycle operating mode and either one of a so-called early intake-valve closing Miller-cycle operating mode at which the intake valve is closed at approximately 90 degrees of crankangle before BDC on the intake stroke and a so-called late intake-valve closing Miller-cycle operating mode at which the intake valve is closed approximately 90 degrees of crankangle after BDC on the intake stroke. The early intake-valve closing Miller-cycle operating mode and late intake-valve closing Miller-cycle operating mode both contribute to a reduction in effective compression ratio. JP7-259655 teaches lowering an exhaust-gas recirculation rate by switching from the standard Otto-cycle operating mode to the Miller-cycle operating mode. That is, JP7-259655 utilizes switching between the standard Otto-cycle operating mode and the Miller-cycle operating mode instead of using a variable compression ratio mechanism (a multi-link type piston-crank mechanism) as disclosed in JP2000-73804 that variably controls a mechanical compression ratio xcex5.
In the spark-ignition internal combustion engine disclosed in JP2000-73804 capable of variably controlling a nominal compression ratio (or a mechanical compression ratio xcex5), it is possible to enhance the thermal efficiency by adjusting compression ratio xcex5 to as high a ratio as possible under a part load condition. However, adjustment of the compression ratio to high results in a rise in combustion temperature, thereby increasing cooling loss. Thus, during part loads, a remarkable improvement in fuel economy cannot be achieved, because, on the one hand, the fuel consumption rate tends to reduce due to the enhanced thermal efficiency, and, on the other hand, the fuel consumption rate tends to increase due to the increased cooling loss. In particular, in case of a spark-ignition internal combustion engine employing a variable compression ratio mechanism that mechanical compression ratio xcex5 is variably adjusted by changing a top dead center (TDC) position of a piston stroke characteristic containing both the TDC position and BDC position with the compression ratio adjusted to high, the TDC position tends to extremely approach to the cylinder head in such a manner as to form an excessively flat combustion chamber. Such an excessively flat shape of combustion chamber leads to an increase in a so-called S/V ratio of the surface area existing within the combustion chamber to the volume existing within the combustion chamber, thus increasing cooling loss. As is generally known, in order to improve fuel economy under a part load condition, it is very effective to add exhaust gases recirculated. Such EGR addition contributes to a reduction in cooing loss but leads to the problem of slow combustion velocities. That is, combustion begins at a late timing after TDC. Such a retardation in combustion results in increased time loss. In particular, during the part-load condition, there is a tendency for combustion to occur slowly, and therefore the thermal efficiency tends to remarkably reduce due to the increased time loss. Additionally, a large amount of exhaust gases recirculated causes unstable combustion.
In the same manner, the previously-noted Miller-cycle, employing early intake-valve closing, contributes to a reduction in pumping loss and cooling loss. However, the early intake-valve closing Miller-cycle operating mode also leads to the problem of slow combustion velocities, that is, the increased time loss.
Accordingly, it is an object of the invention to provide an engine control system of an internal combustion engine with a variable compression ratio mechanism and an exhaust-gas recirculation control system, which avoids the aforementioned disadvantages.
It is another object of the invention to avoid problems of increased time loss and unstable combustion, occurring owing to EGR and early intake-valve closing, by properly setting a piston velocity characteristic (linkage layout) of a multi-link type piston-crank mechanism constructing a variable compression ratio mechanism, and to remarkably improve fuel economy under a part load condition by way of an optimal combination of several ways to improve fuel economy, namely compression ratio control, EGR control, engine valve timing control, and improved linkage layout of the multi-link type piston-crank mechanism.
It is a further object of the invention to provide an internal combustion engine with a variable compression ratio mechanism and an exhaust-gas recirculation control system, capable of effectively properly using internal EGR and/or external EGR.
In order to accomplish the aforementioned and other objects of the present invention, an engine control system for an internal combustion engine comprises a variable compression ratio mechanism comprising a multi-link type piston-crank mechanism having a plurality of links and enabling a compression ratio of the engine to be varied by changing a piston stroke characteristic by way of a change in an attitude of apart of the links, an exhaust-gas recirculation system enabling at least one of external EGR and internal EGR to be controlled, the links of the multi-link type piston-crank mechanism being laid out, so that a piston velocity near top dead center, obtained by the multi-link type piston-crank mechanism, is slower than a piston velocity near top dead center, obtained by a single-link type piston-crank mechanism having at least the same piston stroke as the multi-link type piston-crank mechanism, during a part load condition of the engine, the variable compression ratio mechanism controlling the compression ratio to a predetermined high compression ratio, and during the part load condition, the exhaust-gas recirculation system increasing exhaust-gas recirculation.
According to another aspect of the invention, an engine control system for an internal combustion engine comprises a variable compression ratio mechanism comprising a multi-link type piston-crank mechanism having a plurality of links and enabling a compression ratio of the engine to be varied by changing a piston stroke characteristic by way of a change in an attitude of apart of the links, an exhaust-gas recirculation system enabling at least one of external EGR and internal EGR to be controlled, the links of the multi-link type piston-crank mechanism being laid out, so that a maximum piston acceleration near top dead center, obtained by the multi-link type piston-crank mechanism, is less than a maximum piston acceleration near bottom dead center, obtained by the multi-link type piston-crank mechanism, during a part load condition of the engine, the variable compression ratio mechanism controlling the compression ratio to a predetermined high compression ratio, and during the part load condition, the exhaust-gas recirculation system increasing exhaust-gas recirculation.
According to a further aspect of the invention, an engine control system for an internal combustion engine comprises variable compression ratio means comprising a multi-link type piston-crank mechanism having a plurality of links that enable a compression ratio of the engine to be varied by changing a piston stroke characteristic by way of a change in an attitude of a part of the links, exhaust-gas recirculation means for enabling at least one of external EGR and internal EGR to be controlled, the links of the multi-link type piston-crank mechanism being laid out, so that a piston velocity near top dead center, obtained by the multi-link type piston-crank mechanism, is slower than a piston velocity near top dead center, obtained by a single-link type piston-crank mechanism having at least the same piston stroke as the multi-link type piston-crank mechanism, during a part load condition of the engine, the variable compression ratio means controlling the compression ratio to a predetermined high compression ratio, and during the part load condition, the exhaust-gas recirculation means increasing exhaust-gas recirculation. More preferably, the links of the multi-link type piston-crank mechanism may be laid out, so that a maximum piston acceleration near top dead center, obtained by the multi-link type piston-crank mechanism, is less than a maximum piston acceleration near bottom dead center, obtained by the multi-link type piston-crank mechanism.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.